Various instruments for use in optical tooling are known in the art and used to establish an optical reference line parallel to one axis of the object being worked upon and then to establish optical planes at right angles to the basic reference line. Known optical tooling instruments often require the use of multiple operators and instruments for verification and calibration purposes to insure that a particular "shot" or line of sight remains in a plane parallel to the reference line or plane. Normally, an optical micrometer will need no maintenance beyond cleaning the cover glass on its front and periodic calibration. However, if the micrometer is used in environments with heavy airborne dirt and grime, it usually requires the micrometer to be sent back to the factory for complete disassembly, cleaning and recalibration. Often, the calibration of the optical micrometer may be attempted at the field site. Because of the need of accuracy, however, the calibration procedure should only be attempted by technicians with proper training and experience in the use and calibration of optical measurement equipment.
In calibrating an optical micrometer, specific steps must be followed. Normally, the steps for calibrating an optical micrometer will determine the backlash, zero centering and range accuracy of the micrometer. The backlash test determines the backlash in the sector gear of a conventional optical micrometer. Because correction of the backlash requires complete disassembly of the micrometer, it should not be attempted in the field. Therefore, if after checking for the backlash, the user cannot repeat the readings with less than one half of one minor graduation of backlash in either direction of zero, the micrometer cannot be calibrated accurately. The micrometer must be returned to a repair facility. If the backlash test provides a reading of zero or less than one half of one minor graduation in error, then the micrometer may be calibrated further and then used.
A zero centering calibration test requires mounting micrometer, then placing a bi-filar target about three feet away from the micrometer and at least nominally perpendicular to its line of sight, setting the micrometer calibration drum to zero, then using tangent screws to register the measuring reticle line (the line that moves when the micrometer drum is turned) precisely on the target. The other reticle line should be approximately on the target. The next step is to rotate the micrometer 180 degrees about the test instrument barrel. If needed, the micrometer drum is turned to bring the measuring reticle line back onto the target. If the reading of the micrometer drum is not less than one-half of one minor graduation, then a zero centering of the graduated ring will be required.
In checking the range accuracy of the optical micrometer, the operator will be checking the axial position (range symmetry) and operating radius (range length) of the sector pin. Because changing the sector pin geometry affects the zero centering, this calibration can be very complicated. If a correction to either dimension of the micrometer range is made, then a recalibration of the zero centering is required. Therefore, there is a need for an optical micrometer that can be easily calibrated at the field site without requiring such elaborate calibration steps and direct reading to eliminate backlash.
Accordingly, there is a need for a digital optical micrometer that can be reliably turned on by just pressing a button and that can provide instant calibration of the range and zero centering of the optical micrometer. Further, there is a need for a digital optical micrometer that does not require the backlash test because of the use of a direct reading system of the optical flats position that directly senses the angle of the optical flat window rather than sensing the position of gears that position the window. Additionally, there is a need for a digital optical micrometer that includes a programmable LCD read-out which can be inverted at a touch of a button, an automatic shut-off system and change of units.